The invention relates generally to the field of intermittent combustion engines. More specifically, embodiments of the invention relate to system's for controlling detonation combustor timing input for pulse detonation engines.
A pulse detonation engine (PDE) is a type of intermittent combustion engine that is designed primarily to be used in high-speed, high-altitude environments. The PDE can achieve an efficiency far surpassing gas turbines with almost no moving parts.
Like other jet engines, the PDE takes in air at its front end. In a conventional jet engine, intake air would be driven by a fan through a multistage compressor and into a combustion section or burner where fuel would be burned continuously.
While the operating principle of a PDE is similar to a pulse jet engine, the fundamental difference of a PDE is that it detonates the air/fuel mixture. The pulse jet uses a series of shutters, or careful tuning of the inlet, to force the air to travel only in one direction through the engine to ensure that the mixture exits to the rear thereby pushing an aircraft forward. Another difference is the way in which airflow and combustion in the engine is controlled.
Typically, fuel is consumed either by deflagration, which is slow burning, or detonation, which is a more energetic process. Detonation is inherently more efficient than deflagration. The PDE combustion process burns all of its air/fuel mixture while still inside the engine at a constant volume. While the maximum energy efficiency of most types of jet engines is approximately 30%, a PDE can attain an efficiency near 50%.
All regular jet engines and most rocket engines operate on the rapid, but subsonic combustion of fuel. The PDE is a jet engine that operates on the supersonic detonation of fuel.
One attribute of deflagration is that the flame travels at a speed significantly lower than the speed of sound. However, detonation is a more powerful reaction of the air/fuel mixture and results in such a rapid reaction that the pressure wave created travels at supersonic speeds. Detonation is a violent explosion and produces higher pressures than the process of deflagration.
If a tube were closed at one end and filled with a mixture of air and fuel at the closed end and ignited by a spark, a combustion reaction would propagate down the tube. In deflagration, the reaction would move at tens of meters per second. In detonation, a supersonic shock wave travels the length of the tube at thousands of meters per second. Detonation compresses and ignites the air/fuel mixture almost instantaneously in a narrow, high-pressure heat-release zone.
To create a narrow, high-pressure heat-release zone, the engine must precisely coordinate fuel input, airflow and ignition to create a deflagration-to-detonation transition (DDT). DDT is the process by which an ordinary flame suddenly accelerates into a powerful detonation. While detonation generates more thrust than deflagration for the same fuel consumption, only tiny amounts of fuel can be detonated at a time. For a continuous thrust, many detonations per second are required. For example, the cycle rate of a pulse jet is typically 250 pulses per second. A PDE is thousands of pulses per second.
For a realizable PDE, an air/fuel flow is typically switched between a plurality of combustor tubes, where in each, the air/fuel mixture must detonate cleanly many times per second. The switching is performed using a rotary or cylindrical valve spinning at thousands of revolutions per minute having apertures which alternately open and block airflow to an inlet for each combustor.
A cycle is formed in which the valve admits fuel into a stream of air flowing into a combustor and a spark initiates a DDT producing a shock wave that travels the length of the combustor generating thrust.
PDE designs that use rotary valves to control detonation require a sophisticated synchronization system to ensure that the externally driven valve opens and closes each combustor to inject and detonate the charge at exactly the right moment to optimize detonation of the air/fuel mixture. Detonating the mixture either too early reduces combustion efficiency and if late, the mixture will leave the combustor tube.
The position of the rotary or cylindrical valve is important for the timing of air/fuel mixture and subsequent ignition. One misfire (fuel or ignition at an inappropriate time) can destroy the entire engine. It is therefore desired to have a ±0.5 degree of angular resolution accuracy for the valve when performing ignition timing.